The changes in membrane ionic conductances during excitation have been well characterized. However, the biochemical events within the neuron membrane that give rise to those changes remain largely unknown. Through chemical modification we shall test for the contribution of specific classes of reactive groups to excitation. Recently developed methods will allow alterations of some as yet unexamined residues, including two that have been implicated in recent theories of excitation. Voltage clamp and internal perfusion techniques will allow fine resolution of their modes of action. In further study, the relatively sparse distribution of sodium channels in the membrane demands a higher degree of selectivity. Through a method of prior site protection, the highly specific inhibitor tetrodotoxin (TTX) will first protect groups from modification and will then be removed, to allow selective labeling of channel sites with radioactive, electron-dense, or fluorescent compounds. Studying molecular interactions responsible for native configuration, we shall employ destabilizing conditions ranging from changes in ionic strength to high concentrations of detergents or denaturants, and shall study their effects on both excitation and TTX- binding ability. These perturbations, affecting different modes of chemical binding, will be applied both to intact axons and to nerve homogenates, with the release of the TTX receptor in soluble form as an additional objective. Knowledge of site structure could be of value in understanding interactions of drugs with the nervous system, and could lead to the eventual ability to design neuropharmacological agents, including anesthetics, that would act with a high degree of specificity.